Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, due to a developing shortage, the cost of propylene is likely to increase. Thus, a process that uses higher alkenes instead of propylene as feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenols.
One such process involves the hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Such a process is described in, for example, U.S. Pat. No. 6,037,513.
However, one problem in producing phenol by way of the cleavage of cyclohexylbenzene hydroperoxide is that the cyclohexanone and phenol produce an azeotropic mixture composed of 28 wt % cyclohexanone and 72 wt % phenol. Thus any attempt to separate the cleavage effluent by simple distillation results in this azeotropic mixture. To obviate this problem it has been proposed to integrate the cyclohexylbenzene oxidation and cleavage process with a dehydrogenation step whereby at least part of the cyclohexanone is converted to additional phenol (see International Patent Publication No. WO2010/024975). Such a dehydrogenation step is generally achieved by contacting the cyclohexanone with a supported noble metal catalyst at a temperature of about 250° C. to about 500° C.
For example, U.S. Pat. No. 3,534,110 discloses a process for the catalytic dehydrogenation of cyclohexanone and/or cyclohexanol to phenol over a catalyst comprising platinum and preferably iridium on a silica support. The catalyst also contains 0.5 to 3 wt % of an alkali or alkaline earth metal compound, which, according to column 3, lines 43 to 49, should be incorporated after addition of the platinum since otherwise the resulting catalyst composition has inferior activity, selectivity and life. To obtain high conversion rates, the '110 patent teaches that the dehydrogenation should be conducted at a temperature of 320 to 450° C. and a pressure of 0.5 to 10 kg/cm2.
In addition, U.S. Pat. No. 3,580,970 discloses a process for the dehydrogenation of cycloaliphatic alcohols and ketones to the corresponding hydroxyaromatic alcohols in the presence of a catalyst comprising a Group VIII metal, particularly nickel, and tin in a molar amount of about 1.7 to about 15 moles of Group VIII metal per mole of tin. The catalyst may further comprise an alkali metal stabilizing agent in an amount between about 0.3 to about 10 parts by weight of an alkali metal sulfate per part by weight of the Group VIII metal. The hydrogenation can be conducted at 200 to 500° C., but conversion is said to suffer if the temperature is allowed to decrease below the preferred range of 300 to 450° C.
U.S. Pat. No. 4,933,507 discloses a method of dehydrogenating cyclohexenone to phenol comprising reacting hydrogen and cyclohexenone in the vapor phase in a molar ratio of 0.5 to 4.0 moles of hydrogen per mole of cyclohexenone at a pressure of at least one atmosphere and a reaction temperature of 300° C. to 500° C. using a solid phase catalyst containing platinum, in the range of 0.2 to 10 wt % of the sum of the catalyst plus support, and an alkali metal, in the range of 0.2 to 3.0 calculated in terms of the weight ratio of K2CO3 to platinum, both the platinum and the alkali metal being carried on a support.
U.S. Pat. No. 7,285,685 discloses a process for the dehydrogenation of a saturated carbonyl compound, such as cyclohexanone, in the gas phase over a heterogeneous dehydrogenation catalyst comprising platinum and/or palladium and tin on an oxidic support, such as zirconium dioxide and/or silicon dioxide. In general, the dehydrogenation catalyst contains from 0.01 to 2 wt %, preferably from 0.1 to 1 wt %, particularly preferably from 0.2 to 0.6 wt %, of palladium and/or platinum and from 0.01 to 10 wt %, preferably from 0.2 to 2 wt %, particularly preferably from 0.4 to 1 wt %, of tin, based on the total weight of the dehydrogenation catalyst. In addition, the dehydrogenation catalyst can further comprise one or more elements of Groups I and/or II, preferably potassium and/or cesium, in an amount of from 0 to 20 wt %, preferably from 0.1 to 10 wt %, particularly preferably from 0.2 to 1.0 wt %, based on the total weight of the catalyst. The temperature employed in the dehydrogenation process can range from 300 to 1200° C., preferably from 400 to 600° C.
Research into the cyclohexanone dehydrogenation process has now shown that, although catalyst optimization can allow the production of phenol with good selectivity, typical process conditions result in the coproduction of significant levels of impurities. These impurities include alkylbenzenes, such as t-butylbenzene and n-pentylbenzene, and alkylphenols, such as 2-methyl phenol, as well as heavy products, such as 2-phenyl phenol, diphenyl ether, dibenzofuran and cyclohexyl phenyl ether. Whereas the heavy products result in undesirable yield loss, the alkylbenzenes and alkylphenols pose particular problems since they typically co-boil with or form azeotropic mixtures with phenol. This renders purification of the phenol extremely difficult and expensive. According to the present invention, it has now been found that, by operating the dehydrogenation process at sufficiently mild conditions to lower the cyclohexanone conversion levels to below 50%, the production of impurities and especially alkylbenzenes and alkylphenols can be reduced to levels tolerable in the phenol product.